1. Field of the Invention
This invention relates generally to vehicle transparencies, such as vehicle windshields, and, in one particular embodiment, to a heatable vehicle windshield.
2. Technical Considerations
It has been known to pass electric current through a conductor (e.g., a wire or an electrically conductive coating) on a laminated vehicle windshield in order to raise the temperature of the windshield. This is particularly useful in colder climates for defogging and to melt ice and snow on the windshield.
In wire-heated windshields, fine electrically-conductive wires are placed between the windshield plies. The wires are connected to a power source, such as a conventional 14 volt (v) vehicle alternator. The wires have sufficiently low resistance to provide the windshield with a power density of 5 to 7 watts per decimeter squared (W/dm2).
However, a problem with wire-heated windshields is that the wires can be seen by the vehicle occupants. The presence of these wires can be aesthetically undesirable and can interfere with visibility through the windshield. If the diameter of the wires is decreased to try to reduce the visibility of the wires, the number of wires must be increased to maintain the desired power density. The increased number of wires adversely decreases the total solar energy transmitted (TSET) of the windshield. If the height of the windshield increases, the wires must be longer to maintain the desired power density. These longer wires are also undesirable with respect to windshield aesthetics and/or transmittance.
In an effort to address this problem, some heated windshields utilize transparent, conductive coatings rather than wires to heat the windshield. While these known transparent coatings overcome the aesthetic and visibility problems associated with the use of wires, conventional heated windshield coatings also have some drawbacks. For example, conventional heated windshield coatings typically have a sheet resistance of 2 ohms per square (Ω/□) or greater. A conventional 14v (80 ampere; 1,120 watts) alternator does not provide sufficient voltage to power a conventional heated windshield coating to a temperature sufficient for de-icing. Therefore, for vehicles with these conventional coatings, the vehicles must be altered to increase the available voltage. One way to do this is to replace the conventional 14v (1,120 W) alternator with a 42v (2,500 W to 5,000 W) alternator. Another way to address this problem is to maintain the conventional 14v alternator but add a DC to DC converter to step up the voltage to a sufficient level, e.g., 42v, to heat the windshield to a commercially acceptable level.
However, replacing the conventional 14v alternator or adding a DC to DC converter increases the cost and complexity of the vehicle electrical system. Also, as one skilled in the art will appreciate, all things being equal, a system that functions on 14 volts is typically safer to operate than one that operates on 42 volts.
Therefore, it would be desirable to provide a transparency, such as a laminated vehicle transparency, that reduces or eliminates at least some of the problems associated with conventional heatable transparencies.